Process and installation for pyrolysis of a product in the form of divided solids, in particular polymer waste

ABSTRACT

An installation for pyrolyzing divided solids, such as waste plastics, rubbers, or elastomers. The installation has a device for making divided solids suitable for facilitating pyrolysis by a pyrolysis reactor. The reactor inlet is leak-tightly connected to the preconditioning device and has at least one heating transfer member for transferring the divided solids within the reactor while also pyrolyzing the solids. The member is a conveyor screw or a vibrating tube, and being made of a material that is associated with means for heating it by the Joule effect. The preconditioning device includes liquefaction means for bringing the substance to the outlet of the preconditioning device in a liquid state at an outlet temperature, and a regulator for regulating the temperature of the substance at the outlet from the preconditioning device as a function of a temperature representative of an inlet temperature of the reactor.

The present invention relates to pyrolyzing a substance in the form of divided solids, in particular polymeric waste such as waste plastics materials, rubbers, and/or elastomers.

BACKGROUND OF THE INVENTION

Proposals are made in Document WO 2010/128055A1 for an installation for pyrolyzing a substance in the form of divided solids and comprising:

-   -   a preconditioning device comprising a double-wall auger having a         metering screw and associated with a flow of hot liquid between         the walls for enabling the substance to be brought to a state         that is suitable for facilitating pyrolysis thereof; and     -   a pyrolysis reactor having an inlet connected in leaktight         manner to the outlet of the preconditioning device and fitted         with at least one heating transfer member for transferring the         substance from the inlet to an outlet of the reactor while also         pyrolyzing the substance, the member being for example in the         form of a conveyor screw or a vibrating tube, and being made of         a material that is associated with means for heating it by the         Joule effect.

When used more particularly with polymeric waste such as plastics materials, rubbers, and/or elastomers, it has been found that the yield of the treatment recommended in the above-mentioned document does not enable pyrolysis gas to be obtained that has a high methane content. In addition, the density of the treated substances remains low at the beginning of their treatment, thus leading to limited yields.

OBJECT OF THE INVENTION

An object of the invention is to propose a method and an installation for more effective pyrolyzing of a substance in the form of divided solids, such as polymeric waste.

SUMMARY OF THE INVENTION

In order to achieve this object, the invention proposes an installation for pyrolyzing a substance in the form of divided solids, in particular polymeric waste such as waste plastics, rubbers, or elastomers, the installation comprising:

-   -   a preconditioning device for preconditioning the substance and         for bringing it to a state suitable for facilitating pyrolysis         thereof; and     -   a pyrolysis reactor having an inlet connected in leaktight         manner to the outlet of the preconditioning device and provided         with at least one heating transfer member for transferring the         substance from the inlet to the outlet of the reactor while also         pyrolyzing the substance, the member being in the form of a         conveyor screw or a vibrating tube, and being made of a material         that is associated with means for heating it by the Joule         effect.

According to the invention, the preconditioning device includes liquefaction means for bringing the substance to the outlet of the preconditioning device in a liquid state at an outlet temperature, together with regulator means for regulating the temperature of the substance at the outlet from the preconditioning device as a function of a temperature that is representative of an inlet temperature of the reactor.

The use of liquefaction means for heating, compacting, and liquefying the polymeric substance prior to pyrolyzing it procures numerous advantages:

-   -   introducing the substance into the reactor in liquid form leads         to the substance being densified on introduction into the         reactor, thereby contributing to significantly increasing the         yield of the installation;     -   introducing the substance into the reactor in liquid form makes         it possible to provide a leaktight connection between the         preconditioning device and the reactor that is very simple and         that prevents any oxygen being introduced into the reactor,         thereby avoiding burning pyrolysis gas in the reactor, and         contributing to significantly enriching the methane content of         said gas. Introducing the substance into the reactor in liquid         form makes it possible to omit the sluice valve that is         conventionally interposed between the preconditioning device and         the reactor, since there is no risk of gas penetration; and     -   introducing the substance into the reactor in liquid form at a         temperature that is sufficiently high serves firstly to prevent         excessive cooling in the upstream portion of the reactor,         thereby improving the efficiency of the pyrolysis, and secondly         to prevent any cooling of the inlet of the reactor that might         give rise to the transfer member of the reactor becoming blocked         if the substance were allowed to resolidify.

These advantages lead to a spectacular improvement in the methane content of the pyrolysis gas. Experiments carried out by the Applicant have thus make it possible to obtain methane contents greater than 40%.

The liquefaction means preferably comprise a heating extruder.

The invention also relates to a method of pyrolyzing a substance in the form of divided solids, in particular polymeric solids such as plastics, rubber, or elastomer waste, the method comprising the following successive steps:

-   -   preconditioning the substance in order to bring it into a state         suitable for facilitating pyrolysis thereof; and     -   pyrolyzing the substance as preconditioned in this way by means         of a pyrolysis reactor having an inlet connected in leaktight         manner to an outlet of the preconditioning device and provided         with at least one heating transfer member for transferring the         substance from the inlet to an outlet of the reactor while         pyrolyzing the substance, the device being in the form of a         rotary tubular screw or a vibrating tube, and being made of         material that is associated with means for heating it by the         Joule effect.

According to the invention, the preconditioning step includes liquefaction suitable for bringing the substance, prior to being introduced into the reactor, into a liquid state at a given outlet temperature, with the outlet temperature being regulated as a function of an inlet temperature of the reactor.

BRIEF DESCRIPTION OF THE DRAWING

Reference is made to the sole FIGURE of the accompanying drawing, which is a diagrammatic view of a pyrolysis installation in accordance with the invention, for performing the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The FIGURE is a diagrammatic view of an installation for processing a substance in the form of divided solids, in particular polymeric solids such as plastics, rubber, or elastomer waste. It may be household waste, industrial waste, or indeed substances that have previously been micronized.

In general, the substances concerned are divided solids capable of producing in particular pyrolytic oils by recondensation of gas when the substances are pyrolyzed at temperatures lying in the range 400° C. to 600° C., which oils have energy or chemical content that is suitable for recycling, and the pyrolysis gas also contains components (such as methane, hydrogen, acetylene, . . . ) that are industrially advantageous when the substances are pyrolyzed at temperatures in the range 600° C. to 900° C.

The substance for treatment, referenced P, is initially inserted into a preconditioning device 10 enabling the substance to be compacted and heated so as to liquefy it before it is introduced into a pyrolysis reactor 20.

The present description relates particularly to the preconditioning step, since pyrolysis as such is itself well known. Reference may usefully be made to Document WO 2010/128055A1, which is incorporated herein by reference, for a detailed description of the reactor and of the pyrolysis reaction.

In the present description the preconditioning device is an extruder 10 having a body 11 defining a cylindrical volume in which a wormscrew 12 is mounted to rotate under drive from an associated external motor 13. The body 11 has an inlet 14 connected to a feed hopper 15. The substance P is loaded and stored in the feed hopper 15 which may be provided with a (high and low) level sensor and with a mechanical bridge-breaking system in order to avoid banking and bridging phenomena that would prevent the substance from being removed. The hopper preferably has a force-feed member for forcing the substance into the extruder 10. The extruder 10 is fitted in conventional manner with heater means 17 that enable the substance to be melted progressively so that it leaves the extruder in compacted and liquid form and at a given outlet temperature. For example, the heater means include circulating hot fluid between the two walls of a double-walled casing forming the body 11.

Thus, the substance entering the pyrolysis reactor is already hot, thereby greatly enhancing the temperature rise of the substance inside the reactor.

The substance as heated, compacted, and liquefied in this way by the extruder 10 leaves the extruder via an outlet 16 so as to penetrate into the pyrolysis reactor 20, which has its inlet connected in leaktight manner to the outlet 16 of the extruder. Given the liquid form of the substance entering the reactor, the risk of oxygen penetrating into the reactor is minimal in this configuration, and it is easy to make a leaktight connection between the extruder 10 and the reactor 20 that passes through a sluice valve.

The presently-described pyrolysis reactor 20 is fitted with a core-less conveyor screw 22 that is heated by the Joule effect, said screw serving both to transfer and also to heat the substance traveling through said reactor. In this example, the conveyor screw 22 extends inside a closed enclosure 21 having walls that are preferably made of refractory material. An electrical power unit 100 connected via electrical connections 101 and 102 to two ends of the conveyor screw 22 serve to heat the screw as a function of a setpoint temperature, thereby achieving fine control over the temperature inside the enclosure.

The transfer screw 22 heated by the Joule effect thus drives the substance from upstream to downstream while simultaneously pyrolyzing it, the substance having a transit time in the reactor that is a function of the substance in question, which time lies in particular in the range a few seconds to a few tens of minutes, the transit time being adjusted by acting on the speed of rotation of the conveyor screw 22 as determined by an external motor 26.

According to an important aspect of the invention, the heater means 17 include regulator means for regulating the temperature at which the substance leaves the extruder as a function of a temperature that is representative of the temperature that exists at the inlet to the reactor. In practice, if it is not possible to measure the inlet temperature directly, this inlet temperature, which is necessarily lower than the temperature that exists in the core of the reactor because of edge effects, may be estimated by subtracting substantially one hundred degrees from the setpoint temperature delivered to the electrical power unit 100 for heating the screw.

The purpose of this regulation is to deliver the substance at the outlet from the extruder 10 at a temperature that is naturally higher than the liquefaction temperature of the substance in order to guarantee that the substance leaves in liquid form without any risk of solidifying on penetrating into the reactor, but which temperature must nevertheless remain lower than the inlet temperature of the reactor in order to prevent any premature initiation of the pyrolysis in the extruder 10. Care must then be taken to ensure that the temperature of the substance at the outlet from the extruder 10 is not too far removed from the inlet temperature of the reactor, since that would slow down pyrolysis and give rise to a large temperature gradient inside the reactor. In practice, care is taken to ensure that the difference between the outlet temperature from the extruder and the inlet temperature to the reactor is less than one hundred and fifty degrees Celsius, and preferably less than one hundred degrees Celsius.

The substance as preconditioned in this way in accordance with the invention by the extruder is very easy to pyrolyze in the reactor, and gives rise in particular to pyrolysis gases having a high methane content. It then suffices to recover this gas at the outlet from the reactor, in order to subject it to conventional treatments: storage, condensation to recover the condensable fraction of the gas in the form of pyrolysis oils, delivery to a burner of a heater or of an industrial installation, . . . .

By way of example, for treating very high density polyethylene waste (having its liquefaction temperature lying in the range 100° C. to 200° C.), the following parameters may be used:

-   -   pyrolysis temperature in the core of the reactor: 600° C.;     -   estimated inlet temperature to the reactor: 500° C.;     -   outlet temperature from the extruder: 400° C.; and     -   transit time through the reactor in the range 10 minutes (min)         to 20 min.

The following results are then obtained:

-   -   residual solids ratio: 1% to 5%;     -   oil ratio: 10% to 20%; and     -   gas ratio: greater than 75% with a methane content greater than         40%.

It can thus be seen that combining preconditioning means providing liquefaction at the regulated outlet temperature with a pyrolysis reactor having a heating conveyor screw is particularly advantageous and gives rise to results that are spectacular in terms of the methane content in the pyrolysis gas.

It should be observed that the transit time through the reactor is easily adjustable (typically in the range 5 min to 30 min) by acting on the power supply frequency to the motor 26, while the temperature that exists in the core of the reactor is also easily adjustable by acting on the electrical power delivered to the conveyor screw 22 by means of a static relay or thyristor connected to control the temperature of the screw (typically in the range 400° C. to 850° C.). Naturally, the higher the temperature, the greater the proportion of gas. In contrast, the lower the temperature, with a minim of 400° C. (depolymerization limit), the greater the quantity of oil (condensable gas).

According to a particular aspect of the invention described herein, the pyrolysis residue, in particular coke, is collected at the outlet from the reactor in a container 30 that is connected to the reactor via a leaktight connection. The reactor 30 has a discharge tube 31 with an inlet arranged below the level of coke in the container and through which the pyrolysis gas is recovered. This configuration avoids any introduction of oxygen into the installation from downstream, and makes it possible to avoid having a complex sealing device, such as sluice valves.

The invention is not restricted to the embodiment described above, but on the contrary covers any variant using equivalent means to reproduce the characteristics specified above.

In particular, although the transfer member of the reactor is a conveyor screw that is heatable by the Joule effect, it would also be possible to use a vibrating tube likewise heated by the Joule effect, with the substance then being transferred by a helical tube having a vertical axis.

Although the liquefaction means in the presently-described example comprise a heating extruder, thereby enabling the method of the invention to be operated continuously, it would naturally be possible to use other liquefaction means having a regulated outlet temperature, such as a container that is filled with polymeric waste that is heated in order to liquefy it. A valve positioned between the container and the reactor is then opened in order to cause the substance to penetrate in liquid form into the reactor. 

1. An installation for pyrolyzing a substance in the form of divided solids, in particular polymeric waste such as waste plastics, rubbers, or elastomers, the installation comprising: a preconditioning device (10) for preconditioning the substance and for bringing it to a state suitable for facilitating pyrolysis thereof; and a pyrolysis reactor (20) having an inlet connected in leaktight manner to the outlet (16) of the preconditioning device and provided with at least one heating transfer member (22) for transferring the substance from the inlet to the outlet of the reactor while also pyrolyzing the substance, the member being in the form of a conveyor screw or a vibrating tube, and being made of a material that is associated with means for heating it by the Joule effect; the installation being characterized in that the preconditioning device includes liquefaction means (10) for bringing the substance to the outlet of the preconditioning device in a liquid state at an outlet temperature, together with regulator means for regulating the temperature of the substance at the outlet from the preconditioning device as a function of a temperature that is representative of an inlet temperature of the reactor.
 2. An installation according to claim 1, wherein the regulator means maintain the outlet temperature of the substance from the extruder at a temperature lower than the inlet temperature of the reactor, with a difference that is less than one hundred and fifty degrees Celsius.
 3. An installation according to claim 2, wherein the difference is less than one hundred degrees Celsius.
 4. An installation according to claim 1, wherein the liquefaction means comprise a heating extruder.
 5. A method of pyrolyzing a substance in the form of divided solids, in particular polymeric solids such as plastics, rubber, or elastomer waste, the method comprising the following successive steps: preconditioning the substance in order to bring it into a state suitable for facilitating pyrolysis thereof; and pyrolyzing the substance as preconditioned in this way by means of a pyrolysis reactor having an inlet connected in leaktight manner to an outlet of the preconditioning device and provided with at least one heating transfer member for transferring the substance from the inlet to an outlet of the reactor while pyrolyzing the substance, the device being in the form of a rotary tubular screw or a vibrating tube, and being made of material that is associated with means for heating it by the Joule effect; the method being characterized in that the preconditioning step includes liquefaction suitable for bringing the substance, prior to being introduced into the reactor, into a liquid state at a given outlet temperature, with the outlet temperature being regulated as a function of an inlet temperature of the reactor.
 6. A method according to claim 5, wherein the regulation comprises maintaining the outlet temperature of the substance below the inlet temperature of the reactor, with a difference of less than one hundred and fifty degrees Celsius.
 7. A method according to claim 6, wherein the difference is less than one hundred degrees Celsius.
 8. A method according to claim 5, wherein the liquefaction step comprises hot extrusion. 